Heavy hydrocarbons, e.g. bitumen, represent a huge natural source of the world's total potential reserves of oil. Present estimates place the quantity of heavy hydrocarbon reserves at several trillion barrels, more than 5 times the known amount of the conventional, i.e. non-heavy, hydrocarbon reserves. This is partly because heavy hydrocarbons are generally difficult to recover by conventional recovery processes and thus have not been exploited to the same extent as non-heavy hydrocarbons. Heavy hydrocarbons possess very high viscosities and low API (American Petroleum Institute) gravities which makes them difficult, if not impossible, to pump in their native state. Additionally heavy hydrocarbons are characterised by high levels of unwanted compounds such as asphaltenes, trace metals and sulphur that need to be processed appropriately during recovery and/or refining.
A number of methods have been developed to extract and process heavy hydrocarbon mixtures. The recovery of heavy hydrocarbons from subterranean reservoirs is most commonly carried out by steam assisted gravity drainage (SAGD) or in situ combustion (ISC). In these methods the heavy hydrocarbon is heated and thereby mobilised, by steam in the case of SAGD and by a combustion front in the case of ISC, to flow to a production well from where it can be pumped to the surface facilities. The transportability of the viscous heavy hydrocarbon mixture recovered is conventionally improved by dilution with a lighter hydrocarbon.
Another approach that has previously been adopted to improve the transportability of crude heavy hydrocarbon is to upgrade heavy hydrocarbon mixtures on site prior to transportation to a refinery. Thus a heavy hydrocarbon mixture recovered from a well may be upgraded to form lighter oil having an API of about 20-35 degrees on site and then pumped to a refinery. In such a set up, the upgrading is typically carried out by thermal cracking and/or hydrocracking.
The SAGD and ISC based processes currently used suffer from inherent drawbacks. These include:    (i) diluent is often added to transport the recovered hydrocarbon to refineries therefore large volumes of diluent must be transported and stored at extraction sites;    (ii) if upgrading is used to improve transportability, there is a need to transport significant amounts of fuel and/or hydrogen for use in the upgrading processes to the well site;    (iii) higher levels of asphaltenes are present in the recovered hydrocarbon than non-heavy hydrocarbon and it has little commercial value;    (iv) the use of natural gas for steam generation for SAGD causes high CO2 emissions whereas it has already been recognised in the energy industry that CO2 emissions must be managed better; and    (v) ISC generates vast quantities of CO2 whereas, as above, CO2 emissions must be controlled.
There have been a number of attempts in the prior art to alleviate or minimise the above-mentioned disadvantages of conventional SAGD and ISC processing. US 2006/0042999, U.S. Pat. No. 6,357,526 and WO2012/090178, for example, disclose processes for producing heavy oil by SAGD wherein asphaltenes are separated from the crude heavy hydrocarbon and are used to generate steam and/or hydrogen. Nevertheless a need still exists for recovery processes for hydrocarbon mixtures, and especially heavy hydrocarbon mixtures, which are less demanding in terms of external chemicals required to make the mixture transportable. Methods that additionally reduce the need for steam would naturally be particularly beneficial.
The present inventors have now devised a method of recovering and processing a hydrocarbon mixture wherein at least some of the hydrogen generated from gasification of a part of the recovered hydrocarbon mixture is used in upgrading and another part of the recovered hydrocarbon mixture is used as a diluent in the processing of the recovered hydrocarbon mixture. The method of the present invention is therefore at least partially self-sufficient in terms of hydrogen and also diluent.